1. Technical Field
This invention relates generally to a two-cycle internal combustion engine and method of operation, and more particularly to a two-cycle internal combustion engine having a pair of opposed double-acting piston heads partially defining each cylinder of the engine, and to a method for operating such an engine.
2. History of Related Art
Two-cycle engines generally produce from 50% to 80% greater power output per unit piston displacement at the same speed (depending on scavenging), twice as many power impulses per cylinder per revolution, low-cost for valveless designs, and light weight, as compared with conventional four-cycle engines. The advantages of two-cycle engines are applicable to both spark ignition and compression ignition engines.
An example of a two-cycle internal combustion engine having a plurality of double-acting, circularly oscillating pistons disposed in an annular, or toroidal, structure is described in U.S. Pat. No. 3,580,228 titled, "OSCILLATING INTERNAL COMBUSTION ENGINE", issued May 25, 1971 to Octavio Rocha and Serafin Cano, the co-inventors of the present invention. In that patent, the circularly oscillating pistons of the engine were mechanically controlled so that an opposed set of pistons moved toward and away from a fixed point in a respective cylinder, at equal synchronized rates of travel, so that each piston of the opposed set was always simultaneously displaced the same distance from the fixed point in the cylinder.
Also, in the above-referenced two-cycle engine arrangement, the exhaust ports were opened first during an expansion stroke, but closed last during a compression stroke. This characteristic is common to all valveless two-cycle engines wherein the port that opens first during one stroke, closes last during a return stroke. This action precludes effective charging and/or supercharging of the engine to increase the amount of charge per cycle above that of a normally scavenged and charged cylinder. The better a cylinder is charged with air or combustible mixture, the higher the power developed.
Furthermore, in conventional two-cycle engines, such as the above-described circularly oscillating piston engine, it is commonly accepted that combustion occurs instantly when the piston or pistons are at top dead center, i.e., when the gas volume is at a minimum. In actual practice, the minimum gas volume is typically maintained over only a very short time after ignition occurs, after which the combustion event continues during the expansion, or power, stroke which generally results in the higher hydrocarbon emissions typical of two-cycle engines. Hydrocarbon emissions are produced because the flame of combustion cannot completely reach the walls of the cylinder, thus leaving a layer of unburned fuel on the wall and discharging unburned fuel, together with the products of combustion, through the exhaust port of the engine. The flame front is cooled by the cool walls of the cylinder as it approaches the walls, flame speed decreases as the flame front temperature drops, and finally the flame stops. Accordingly, the lower the temperature and pressure of the combustion process, the thicker the unburned layer and the higher the exhaust hydrocarbon emissions. Lean fuel-air mixtures are commonly used to reduce hydrocarbon emissions. However, as the fuel-air ratio is reduced, the flame speed decreases and finally becomes so slow that the combustion process is not completed during the expansion stroke. Therefore, when the minimum volume in the cylinder is maintained over only a very short period of time, the use of lean fuel-air mixtures is restricted.
Another common characteristic of conventional two-stroke engines is uneven cycle pressure peaks between the cylinders of the engine, and even in the same cylinder from stroke to stroke. This characteristic reduces overall engine efficiency, and makes it difficult to produce consistent power output for a given fuel charge.
The present invention is directed to overcoming the problems set forth above. It is desirable to have a two-cycle internal combustion engine that has a relatively long combustion period during which minimum volume conditions are maintained in the combustion chamber so that after ignition, the combustion process starts, and since the minimum volume is maintained and combustion progresses, the pressure greatly increases, whereby lower emissions of hydrocarbons are produced and fuel economy is increased as a result of more efficient combustion during engine operation. It is also desirable to have a two-cycle internal combustion engine in which a pair of opposed piston heads associated with each cylinder are moved at different rates of travel to provide a relatively long minimum volume time and relatively long scavenging times during engine operation. Furthermore, it is desirable to have a valveless two-cycle internal combustion engine in which the exhaust ports open first during an expansion stroke and close first during a compression stroke, thereby enabling effective blow-down, scavenging, charging, and if desired, supercharging at appropriately beneficial times during the operating cycles of the engine. It is also desirable to have a two-cycle engine in which successive cycle pressure peaks evenly, between all cylinders and from stroke to stroke, are substantially uniform for equal fuel charges.